Overview

Brief Summary

History in the United States

Canada thistle was introduced to the United States, probably by accident, in the early 1600s and, by 1954, had been declared a noxious weed in forty three states. In Canada and the U.S., it is considered one of the most tenacious and economically important agricultural weeds, but only in recent years has it been recognized as a problem in natural areas.

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Distribution

Range and Habitat in Illinois

Canada Thistle is a common plant that occurs primarily in central and northern Illinois (see Distribution Map). It is apparently less common or absent from many areas of southern Illinois, although it could be spreading southward. Contrary to the common name, this plant is originally from Eurasia. Typical habitats include cropland, abandoned fields, areas along roads and railroads, vacant lots, weedy meadows, and degraded prairies. This plant can invade lawns that are not mowed regularly, and it is aggressive enough to invade many natural habitats.

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Canada thistle is native to southeastern Europe and the eastern Mediterranean area, and was probably introduced to North America in the 1600s as a contaminant of crop seed and/or ship's ballast [152]. It is probably the most widespread of all thistle species [152]. In addition to North America, Canada thistle is invasive in northern and southern Africa, the Middle East, Japan, India, New Zealand, Australia, and South America. It infests at least 27 crops in 37 countries and thrives in temperate regions of the northern hemisphere [146]. In North America, Canada thistle occurs from Alaska east to the Northwest Territories, Quebec, and Newfoundland and south to California, New Mexico, Kansas, Arkansas, and North Carolina [107]. The PLANTS database provides a map of Canada thistle's distribution in the United States.

Canada thistle has been identified as a management problem in many national parks and on The Nature Conservancy preserves in the upper Midwest, the Great Plains states, and the Pacific Northwest [214]. It is an invader in Mesa Verde National Park, Colorado [67], Yellowstone National Park, Wyoming [4,48,218], Wood Buffalo National Park, Northwest Territories, Canada [83,237], Theodore Roosevelt National Park, North Dakota [32], and the Camas Swale Research Natural Area in the Willamette Valley, Oregon [43].

Although Canada thistle is not usually found in undisturbed forested areas, it has the potential to colonize a wide variety of forest habitats within its range following overstory removal and soil disturbance. The following listings take this potential into account.

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Distribution in the United States

Canada thistle is distributed throughout the northern U.S., from northern California to Maine and southward to Virginia. It is also found in Canada, for which it was named. Canada thistle has been identified as a management problem on many national parks and on preserves of The Nature Conservancy in the upper Midwest, Plains states, and the Pacific northwest.

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Distribution and Habitat in the United States

Canada thistle is an extremely widespread weed of agricultural and ecological areas in the U.S, occurring throughout the northern states and Southwest but is largely absent in the South from Texas to Georgia. Twenty large national parks across the country report it as a serious invasive plant affecting natural resources. It invades a variety of dry to moist open habitats including barrens, fields, glades, grasslands, pastures, stream banks, wet meadows, wet prairies, and open forests. It is not very tolerant of shade.

Physical Description

Morphology

Description

Canada thistle is a perennial introduced forb. It is distinguished from other thistles by creeping horizontal lateral roots, dense clonal growth, and dioecious habit [49,121,240]. Descriptions and terminology of Canada thistle biology can be confusing or contradictory. For example, descriptions of leaf morphology, stem height, and number of flowering heads may differ somewhat between floras. The following discussion provides ranges of what may be encountered for these characteristics, which will vary under different field conditions. Donald [55] and Moore [150] provide comprehensive reviews of the biology of Canada thistle.

Canada thistle has a deep and wide-spreading root system with a slender taproot and far-creeping lateral roots. It often forms large patches, and individual clones may reach 115 feet (35 m) in diameter [55,75,126,186,248]. Most Canada thistle roots are in the top 0.7 to 2 feet (0.2-0.6 m) of soil, but roots can extend as deep as 6.5 to 22 feet (2-6.75 m) [113,152,157]. Carbohydrate reserves are stored in roots and can range from 3% of root fresh weight during spring to as high as 26% in late fall [137]. Roots are injured when directly exposed to freezing temperatures for 2 hours at -5 °C and killed after 2 hours at -7 °C [192]. Arbuscular mycorrhizal infection of Canada thistle roots has been observed in several studies [17,50,116]. Canada thistle does not form rhizomes, despite this assertion in some literature. Adventitious root buds that may form new adventitious shoots can develop along the root at any location, and at any time of the year with favorable growing conditions [55,85]. New plants can also form from root fragments as short as 0.2 inch (6 mm) [157]. Soil type, structure and horizonation may impact the anatomy, morphology and distribution of Canada thistle roots as well. This suggests that root morphology and distribution are site specific and greenhouse studies of root morphology may not apply [55].

While allelopathy has not been conclusively demonstrated for Canada thistle, this species may produce phytotoxins that inhibit the growth of other plants [55,203]. Fructan metabolism in Canada thistle adds to its competitive advantages by allowing it to grow at relatively cool temperatures [37].

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Description

Canada thistle is an herbaceous perennial with erect stems 1½-4 feet tall, prickly leaves and an extensive creeping rootstock. Stems are branched, often slightly hairy, and ridged. Leaves are lance-shaped, irregularly lobed with spiny, toothed margins and are borne singly and alternately along the stem. Rose-purple, lavender, or sometimes white flower heads appear from June through October, generally, and occur in rounded, umbrella-shaped clusters.

The small, dry, single-seeded fruits of Canada thistle, called achenes, are 1-1½ inches long and have a feathery structure attached to the seed base. Many native species of thistle occur in the U.S., some of which are rare. Because of the possibility of confusion with native species, Canada thistle should be accurately identified before any control is attempted.

Flowers, fruits and seeds: flowering occurs in late June to August; flowers are purple to white and about 1 in. long by ½ in. across; seeds are called achenes, are 1-1½ in. long and have a feathery pappus.

Look-alikes: a number of native and exotic thistle species, some which are very rare. Numerous species of thistle occur in North America. Some are invasive, some are native, and most are dependably difficult to distinguish without assistance.

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Ecology

Habitat

Range and Habitat in Illinois

Canada Thistle is a common plant that occurs primarily in central and northern Illinois (see Distribution Map). It is apparently less common or absent from many areas of southern Illinois, although it could be spreading southward. Contrary to the common name, this plant is originally from Eurasia. Typical habitats include cropland, abandoned fields, areas along roads and railroads, vacant lots, weedy meadows, and degraded prairies. This plant can invade lawns that are not mowed regularly, and it is aggressive enough to invade many natural habitats.

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Habitat characteristics

Temperature: Canada thistle grows best between 32 and 90 degrees Fahrenheit (0-32 °C) [150,152]. Extended periods with temperatures over 90 degrees Fahrenheit (32 °C) reduce plant vigor and generally limit growth. High temperatures and shorter days keep Canada thistle from thriving in the southern U.S. Optimum day/night temperatures for growth are 77 and 59 degrees Fahrenheit (25 and 15 °C), respectively [85]. The northern limit of Canada thistle's growth corresponds to the 0 degrees Fahrenheit (-18 °C) mean January isotherm; flowering is also limited in the northern latitudes [150]. Canada thistle invasion of native rangelands appears to be a problem especially of highly productive, mesic habitats [179,203,252]. However, Canada thistle was able to infest subalpine fir/twinflower habitats in western Montana [68]. The temperature exposure of overwintering buds required to reduce survival of Canada thistle was 2 hours at 19 degrees Fahrenheit (-7 °C) and to reduce total dry weight was 2 hours at 23 degrees Fahrenheit (-5 °C) [192]. The ability of adventitious root buds to withstand freezing depends on their location in the soil profile [55,192]. In soil samples from a mid-boreal wetland subjected to increased temperatures, Canada thistle seedling emergence increased significantly (p<0.05) at higher temperatures [99,100].

Moisture: Canada thistle tolerates annual precipitation ranging from 12 to 40 inches (305-1015 mm) per year, and grows best with 16 to 30 inches (400-750 mm) of precipitation per year [83,150,152]. In range and pastureland, Canada thistle is often restricted to swales or other areas of deep, moist soils [128]. Canada thistle is concentrated in disturbed areas and along streams, rivers and other moist areas in Rocky Mountain National Park, although individual plants have been found on relatively dry, sagebrush-dominated sites [139]. A high water table limits root growth [185], but Canada thistle often occurs in wetlands where water levels fluctuate, and in degraded sedge meadows it may be found growing on tussocks elevated above the normal high water line. In a mid-boreal wetland subjected to drought, Canada thistle increased 5- to 13-fold over predrought levels [98,100]. Canada thistle survives well in dry places [185] and under extended periods of drought, but biomass and number of root buds decrease after several years [194]. Growth was increased by high relative humidity (90-100%) over low relative humidity (30-50%) [104].

Elevation and slope: Canada thistle occurs over a wide range of elevations from sea level [58] to elevations in excess of 8,000 feet (2,500 m) [49]. In the northern Rocky Mountains, it is found mainly by roadsides and other disturbed sites in the lower elevations and warmer, drier habitats, and escapes to undisturbed sites at upper elevations [140,235]. In Yellowstone National Park, Wyoming, Canada thistle occurs at elevations ranging from 5,970 to over 7,875 feet (1,820-2,400 m) [4]. In Rocky Mountain National Park, Colorado, Canada thistle coverage is greater at elevations around 8,375 feet (2,550 m) and decreases at elevations around 9,095 feet (2,770 m), but occurs up to at least 9,185 feet (2,800 m) [139]. Canada thistle grows best on shallow (9-30%) slopes [4,140].

Soils: The wide distribution of Canada thistle suggests that it is adaptable to many soil types [55,185]. It grows on all but waterlogged, poorly aerated, and peat soils, including clay, clay loam, silt loam, sandy loam, sandy clay, sand dunes, gravel, limestone, and chalk [161]. Rogers [185] suggests that Canada thistle grows best on limestone soils with abundant moisture. Some authors suggest that it is best adapted to clay soils [152]; others suggest that it prefers well-aerated soils [150]. Preliminary results in Rocky Mountain National Park indicate that soils supporting Canada thistle tended to have a surface (0-10 cm) texture higher in clay and silt than in sand [139]. Canada thistle was found growing on heavily saline soils in central Alberta, though it was absent from saline areas of Saskatchewan and Manitoba [24]. Hardpans, gravel, sand, or very alkaline soil horizons can limit root development of Canada thistle [185].

Competition and light: Canada thistle grows best in open sunny sites [150]. Canada thistle seedlings are much less competitive than established plants, and will survive only if competition is limited and the daytime light intensity remains above 20% of full sunlight [152]. In Rocky Mountain National Park, total canopy cover of vegetation within Canada thistle patches is less than outside the patches [139]. At Yellowstone National Park, Canada thistle was found in 6 out of 10 campgrounds, with occurrences most frequent under a canopy cover of less than 20%, although it was occasionally present under more closed canopy covers (up to 95%) suggesting that it is somewhat tolerant of shade. Twenty percent of the quadrats in which Canada thistle was present had no evidence of disturbance [4]. Because Canada thistle is relatively shade intolerant, it may be found growing along the edges of woods (both deciduous and coniferous), but is rarely found under forest canopy, in undisturbed prairies, good to excellent pastures, or woodland or sites that are shaded most of the day [83,105,161]. In the Delta Marsh in Manitoba, Canada thistle is present in communities dominated by common reed. It is capable of persisting on undisturbed plots, growing with stunted spindly stems and no flowers, but growth improves after disturbance [213].

Generally, Canada thistle establishes and develops best on open, moist, disturbed areas, including ditch banks, overgrazed pastures, meadows, tilled fields or open waste places, fence rows, roadsides, and campgrounds; and after logging, road building, fire and landslides in natural areas [4,45,106,115,122,138,158,163,188,193,216,220]. Roads, streams and ditches provide areas of disturbance and corridors for invasion. At Yellowstone National Park, Canada thistle was found in all levels of disturbance (along horse and foot trails, roadways, and campgrounds) and its abundance increased as disturbance cover increased [4,219]. Physically disturbed habitat in fragmented old growth in Indiana facilitated invasion by exotics including Canada thistle [26]. Canada thistle invasion was also enhanced by heavy grazing by bison [237], areas left barren during planting operations, and on earth mounds made by pocket gophers and badgers in North and South Dakota [93].

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Habitat in the United States

Canada thistle grows in barrens, glades, meadows, prairies, fields, pastures, and waste places. It does best in disturbed upland areas but also invades wet areas with fluctuating water levels such as streambank sedge meadows and wet prairies.

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Faunal Associations

The flowers of Canada Thistle attract a wide variety of insects, including long-tongued bees, short-tongued bees, Sphecid wasps, Vespid wasps, miscellaneous flies, butterflies, skippers, and beetles. Both nectar and pollen are available as floral rewards. The caterpillars of the butterfly Vanessa cardui (Painted Lady) feed on the foliage, as do the caterpillars of many moth species (see Moth Table). Others insects that feed on the foliage, roots, seeds, and other parts of Canada Thistle include leaf beetles, weevils, stink bugs, aphids, treehoppers, the larvae of fruit flies, and grasshoppers; the Insect Table lists many of these species. The seeds of Canada Thistle and other thistles are a source of food to some songbirds, including the Eastern Goldfinch, Pine Siskin, Slate-Colored Junco, Indigo Bunting, and Clay-Colored Sparrow. Because of the thorny foliage, mammalian herbivores usually avoid eating this plant, except when little else is available. If the foliage is eaten, they can have problems with irritation of their mouthparts and digestive tract as a result. Comments

Fire Management Considerations

Abundant evidence of postfire establishment of Canada thistle [16,138,163,193] suggests that managers need to be aware of this possibility, especially if a known seed source is in the area, and take measures to prevent the establishment of Canada thistle after prescribed burning and wildfires. Seeding with aggressive, introduced grasses such as crested wheatgrass, intermediate wheatgrass, orchardgrass, and smooth brome following a prescribed burn in Utah pinyon-juniper communities prevented establishment of Canada thistle, whereas unseeded areas supported Canada thistle seedlings [77]. Similarly, in disturbed forest sites where Canada thistle becomes established, it may be shaded out over time as trees reestablish [56].

Research in this report suggests that response of Canada thistle to fire is variable and it depends on vegetation and site characteristics, as well as frequency, severity and season of burning. Prescribed burns may be effective at stimulating growth of native species and thereby discouraging the growth of invasives such as Canada thistle [182], and may be best if timed to emulate the natural fire regime of a site [44]. Hutchison [105] states that prescribed burning is a "preferred treatment" for the control of Canada thistle, and that late spring burns effectively discourage this species, whereas early spring burns can increase sprouting and reproduction. During the first 3 years of control efforts, he recommends that burns be conducted annually [105], though it is unclear what evidence these recommendations are based on. Season of burn is an important consideration for prescribed burning, as the timing of the burn will determine species composition and cover in the post-fire community [101,102]. Dormant season burning may be a preferred treatment method in some areas, because in many habitats it stimulates growth of native vegetation that subsequently competes with Canada thistle [252]. However, dormant season burning may not be as effective as late spring burning [105]. Controlled studies comparing the effects of these variables in different natural areas are currently lacking in the literature.

Equations for estimating fuel loading of forb communities including Canada thistle are available [27].

The USDA Forest Service's "Guide to Noxious Weed Prevention Practices" [224] provides several fire management considerations for weed prevention in general that apply to Canada thistle. To prevent invasion after wildfires and prescribed burns, re-establish vegetation on bare ground as soon as possible using either natural recovery or artificial techniques as appropriate to site objectives. When reseeding burn areas, use only certified weed-free seed. Monitor burn sites and associated disturbed areas after the fire and the following spring for emergence of Canada thistle, and treat to eradicate any emergent Canada thistle plants. Regulate human, pack animal, and livestock entry into burned areas at risk for weed invasion until desirable site vegetation has recovered sufficiently to resist weed invasion.

When planning a prescribed burn, preinventory the project area and evaluate cover and phenology of any Canada thistle present on or adjacent to the site, and avoid ignition and burning in areas at high risk for Canada thistle establishment or spread due to fire effects. Avoid creating soil conditions that promote weed germination and establishment. Discuss weed status and risks in burn rehabilitation plans. Wildfire managers might consider including weed prevention education and providing weed identification aids during fire training; avoiding known weed infestations when locating fire lines, monitoring camps, staging areas, helibases, etc., to be sure they are kept weed free; taking care that equipment is weed free; incorporating weed prevention into fire rehabilitation plans; and acquiring restoration funding. Additional guidelines and specific recommendations and requirements are available [224].

Broad-scale Impacts of Plant Response to Fire

Several studies have indicated the presence of Canada thistle in burnedareas where it was absent from the prefire community and/or adjacent unburned areas (e.g., [138,158,163]). In Grand Teton National Park, Wyoming, Canada thistle did not occur in unburned forestand was not part of the initial postfire vegetation after a mixed-severitywildfire. It established 2 years after fire on a moderate-severity site, and 9 yearsafter fire on a severe site. On both sites, it decreased to <1% cover bypostfire year 17 as cover of tree saplings increased [56]. Seedlings were foundin a red pine forest inMinnesota, 3 years after fire, but not on adjacent unburned forest [3]. Canada thistleestablished 3 yearsaftermixed-severity fires in sedge meadows in Glacier National Park [242]. In Yellowstone National Park, Canada thistle established after 1988 fires and increased in density over time, 2 to 5 yearsafter fire, in all burnseverities. Density was lowest in the low-severity burns and highest in the stand-replacing burns [219]. Canada thistle established on both bulldozer lines and burned areas after a 1988wildfire in Glacier National Park, but was not present in comparable undisturbedsites [16].

Response of established Canada thistle plants to fireis unclear, as there are mixed reports in the literature. A Canada thistle clone in a mid-boreal wetland sitewas not noticeably changed when burned in the spring with a propane torch tosimulate both light and deep burns [98]. The authors concluded that there is amoderate to high probability that Canada thistle and otherEurasian xerophytic species will dominate these wet-meadows in the short termafter fire, and that they will continue to dominate small areas for longerperiods [100]. There were no significant differences (p<0.05) in Canada thistle cover after springburning in the prairie pothole region of Iowa [145]. In Mesa VerdeNational Park, Colorado, populations of Canada thistle that were well established beforean August wildfire resprouted immediatelyafter the burn, and spread downstream in the canyons. Canada thistle and othernon-native species (e.g., cheatgrass (Bromus tectorum) and musk thistle (Carduusnutans)) continued to dominate the severely burned areas andexpanded their area by 260% 6 years after the wildfire [64,67]. In a native mixed-grass prairie in NorthDakota, late-spring and late-summer burning increased seed production and seedlingnumbers in Canada thistle, but fewerthistles were observed during the years following the burn than before orduring the year of the burn [201]. Dormant season (winter and early spring) burning in eastern Oregon resulted in fewertotal and fewer functional flowerheads on reproductive shoots of Canada thistlewhen compared to unburned control. Also, Canada thistle plants on burned sites grewmore slowly and associated vegetation was more productive than on control sites.It was concluded that burning reduced the relative abundance of Canada thistle and may be useful as a means ofhalting its invasion or spread by maintaining a productive stand of nativevegetation [252]. The discrepancy in these reports is probably due to thelarge number of variables that can affect the response of Canada thistle tofire, including fire severity, for which we lack a standard nomenclature in theliterature. Other important variables include vegetation and site characteristics,frequency, and season of burning.

Site differences such as soil moisture content, plantcommunity, and slope aspect can influence fire severity and may influence theresponse of Canada thistle to fire. In a northwestern Minnesota prairie site,prescribed burning on a nearly level mesic site in badly disturbed prairie hadno effect on Canada thistle flowering while flowering was inhibited on a level,wet-mesic site in badlydisturbed prairie after burning [170].On a forested site in western Montana that was harvested and burned, Canada thistle seems to have increased withboth light and severe burning in the fall, with larger increases on southaspects compared with others [122]. Olson [162] provided evidencethat prescribed burning in the spring either reduced or did not change canopycover of Canada thistle in Minnesota. Results differed between sites, whichdiffered primarily in plant community type and in time and frequency of burning.

Frequency, severity and season of burning may have a considerable effect on Canada thistle response. In a study conducted on a mesic tallgrass prairie site in Colorado, plots that were burned frequently (5 times over 7 years) had lower density of Canada thistle than did and area that was burned only twice during the same period. Results were inconclusive, however, since the final season of the study saw increased spread of Canada thistle from the surrounding area, probably due to clonal growth from existing plants [151]. Similarly,observations in tallgrass prairie sites in South Dakota indicate that late springprescribed burning (when native species are still dormant) on a 4 to 5 year rotation (as per the historic fire regime)encourages the growth of native plants and discourages the growth of Canada,bull and musk thistles. Livestock use must be carefully timed following burning,since grazing early in the growing season can potentially negate beneficialeffects of prescribed fire [44]. However, cover ofCanada thistle was essentially unchanged after 5 years of annualspring burning in mid- to late April, with fires of low to moderate severity, in a prairie site at Pipestone National Monument, Minnesota [15]. On acommon reed marsh in Manitoba, Canada thistle response to burningvaried with season of burn. Aboveground biomass, stem density, and seedlingdensity were unchanged on springburns, but increased on both summer and fall burns [213].Results are presented below:

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Plant Response to Fire

Canada thistle is slightly damaged to enhanced by fire [252]. It is likely to survive fire and sprout vegetatively from its extensive perennial root system (e.g., [64,67,98,145,151,213,252]) (also see Asexual reproduction), or colonize bare ground via seedling establishment after fire [3,56,122,138,158,190,219,242]. For example, in Yellowstone National Park, Canada thistle is rare in unburned forests but locally abundant in burned areas [48]. When sites supporting Canada thistle are burned, its response is variable, and may be affected by season of burn, burn severity, site conditions, and plant community composition and phenology before and after the fire. Existing research provides no clear correlations with these variables.

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Fire Ecology

Canada thistle is adapted to both survive fire on site, and to colonize recently burned sites with exposed bare soil. The extensive root system gives it the ability to survive major disturbances as observed, for example, at Mt. St. Helens, where Canada thistle was part of the initial community after the 1980 eruption. It survived landslide and resprouted from root and stem fragments after the blast [2,45,216]. It is likely to survive fire and sprout vegetatively from its extensive perennial root system (see Asexual reproduction), as was observed, for example, after an August wildfire in Mesa Verde National Park [64,67]. Additionally, there are numerous examples from the literature where Canada thistle seedlings established anywhere from 2 to 9 years after fire [3,56,122,138,158,190,219,242], presumably from wind-dispersed seed, although this is not always clear in the literature.

Canada thistle may change the fire ecology of the site in which it occurs by its abundant, flammable aboveground biomass. For example, in boreal wet-meadows, investigators suggest that Canada thistle has the potential to increase fire frequency and perhaps severity as a result of its abundant and readily ignited litter [100].

The following table provides some historic fire regime intervals for habitats in which Canada thistle may occur:

Successional Status

Canada thistle is an early successional species that emerges from seed or root fragments shortly after disturbance. It grows best in open sunny sites, though may be somewhat tolerant of shade (see "Site Characteristics"). Canada thistle may establish in natural areas as part of the initial plant community after logging [106,109,160,250], fire [16,80,138,163,188,193,242], volcanic eruption (debris deposit, landslide) [2,45,216,217], grazing [143], and road building [140]. Canada thistle and other introduced species are taking over large tracts of logged, burned, or otherwise disturbed land in British Columbia [220]. In northern Idaho, Canada thistle establishes following clearcutting with soil displacement. With low soil displacement, the plant community follows a successional sequence that favors the eventual establishment of tree and shrub species, but with heavy soil displacement, a persistent forb-rich community, including Canada thistle, develops with few tree species present, and very little species replacement over time [106]. Canada thistle may not establish immediately after logging and fire disturbances, but may be delayed for 2 or more seasons [3,56,164,242]. Canada thistle was among the 3 most common species to survive a debris deposit created by the 1980 eruption of Mount St. Helens, where it sprouted from transported root fragments, and from seed [2,45,216]. Canada thistle is also found among the emergent vegetation after drawdown in the Delta Marsh, Manitoba [142,229]. In a study comparing possible control methods for perennial pepperweed, Canada thistle established, along with cheatgrass, after disking and herbicide treatments that reduced cover of native forbs and grasses [112].

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Regeneration Processes

Canada thistle reproduces both sexually by seed and vegetatively by creeping roots. Generally, vegetative reproduction contributes to local spread and seeding to long distance dispersal. Introduction into new areas is mostly by wind- or water-borne seed, or by seed in contaminated crop seed, hay or machinery [55,105]. Canada thistle allocates most of its reproductive energy to vegetative propagation, and a patch can spread rapidly by vegetative means under favorable conditions. Total allocation of dry weight to sexual reproduction was only 7% for Canada thistle grown in pots [23]. However, the contribution of sexual reproduction to the survival and spread of Canada thistle may be underestimated and may be an important mechanism for initiating continued genetic diversity in a clonal population [89].

Sexual reproduction: Shoot elongation and flowering in Canada thistle are induced by 15-hour day length, therefore flowering and seed production will be limited or prevented in regions with shorter summer days [84]. A typical Canada thistle shoot may produce 32 to 69 flowerheads per shoot (1-5 per branch) on average, but can produce as many as 100 flowerheads in a season [150,152]. Canada thistle is "imperfectly dioecious" [55], with male and female flowers occurring on separate plants. Up to 26% of "male" plants are actually self-fertile hermaphrodites or subhermaphrodites that occasionally produce seed [108].

Seed production: Canada thistle is insect pollinated, primarily by honeybees [55,105,150]. Male and female plants must be located within a few hundred yards of each other for insect pollination and seed set to occur [84]. Seed set is highest when male and female plants are intermixed and decreases when female plants are more than 164 feet (50 m) from male plants [125]. Since Canada thistle can grow in large patches, it is not uncommon to find sterile heads of female flowers [125,152]. Canada thistle has a reputation for producing few viable seeds, but the literature gives a wide range of estimates for seed production with numbers ranging from 0 to 40,000 seeds per stem [38,89]. Reports of average seed-set per flowerhead range from 21-93 [89,152]. Kay [108] reports that females produce an average of 30 to 70 seeds/flowerhead and males average 2 to 10 seeds/head. The number of flowerheads per stem reported ranges from 0 to 100 [89]. In annual grasslands in northern California where biomass of Canada thistle was 13+ 8 g/m2, seed production was 1300 seeds/m2, seed rain was 80+ 50 seeds/m2, and germinable seeds in the top 2 cm of soil were 280+110/m2 [96]. Inefficient pollination and genetic variability may contribute to poor seed yields [89]. Seeds of Canada thistle are subject to predation by insects before dispersal, but information is more qualitative than quantitative [55,89]. Weather extremes (cool and moist or hot and dry) can interfere with pollination, so some years even female plants do not produce much seed [61].

Seed dispersal: Canada thistle seeds are released about 2-3 weeks after pollination [123]. They are equipped with a pappus, loosely attached to the seed tip, that enables wind dispersal, and have good aerodynamic efficiency [198]. Canada thistle seeds have been observed windborne on the prairie several hundred meters from the nearest source population [175]. Evidence from seed rain studies on Mount St. Helens, Washington suggests that Canada thistle seeds can travel several kilometers [249]. This dispersal mechanism accounts for the numerous examples of Canada thistle seedling establishment after disturbance in natural areas [45,106,109,216,220], especially after fire [138,163,188,193]. However, wind dispersal has not been considered a major factor in its spread, since the pappus readily breaks off, leaving the achenes within the seedheads [23]. In developed areas, seeds are more commonly spread by animals, in hay, contaminated crop seed, machinery, and irrigation water [161]. Observations in Rocky Mountain National Park indicate that trails, especially those used by horses, are major invasion pathways for Canada thistle [139]. Livestock consuming unprocessed hay before entering national forests will likely spread more Canada thistle seeds than those consuming feed pellets, since pellet manufacturing destroys 99% of viable Canadian thistle seed when it includes grinding and screening [35].

Viability and germination: Canada thistle seeds mature quickly and most are capable of germinating 8 to 11 days after the flowers open, even if the plants are cut when flowering. Moore [150] summarized research indicating that almost all Canada thistle seed can germinate upon dispersal, although germination is extremely variable (0-95%). Viability of seeds during the 1st season after dispersal may be as high as 90% [84]. Most seeds germinate in the spring after the year in which they are produced [97,188], with some seeds producing basal leaves before winter and emerging to flower the next spring [105]. However, Heimann and Cussans [89] indicate that seedlings are not always able to survive the winter. Germination may be affected by ecotype, temperature, day length, depth of seed burial, substrate stratification, and seed freshness [161]. Seeds from "male" plants are smaller and percent germination is lower [108]. Temperature requirements for germination were summarized by Moore [150]; the effects of light, pH, and salinity are summarized by Donald [55]. Canada thistle seeds germinate best in warm temperatures (68 to 104 degrees Fahrenheit (20-40 °C)), with alternating light and dark periods [22,188,245]. Germination in Canada thistle was best after 0.5 to 16 days at 88 to 108 degrees Fahrenheit (31-42 °C) [212]. At lower temperatures germination is aided by high light intensity [89,97]. Germination at higher temperatures can help ensure that maximum germination takes place during warmer periods of the year [89]. Canada thistle seeds are somewhat tolerant of heat, and some were still viable after 10 minutes at 216 degrees Fahrenheit (102 °C) and 2 minutes at 504 degrees Fahrenheit (262 °C), although viability was decreased at these temperatures compared to unheated controls [212]. Canada thistle seeds germinate over a wide range of soil moisture [245]. Heimann and Cussans [89] provide a summary indicating that Canada thistle seed can germinate on the soil surface, but that germination is best when seeds are buried 0.2 to 0.6 inch (0.5-1.5 cm) deep. Emergence as deep as 6 cm in some soil types has been reported [245]. Most germination studies have been done under artificial conditions, and factors influencing germination in the field are far more complex [89].

Seed banking: The soil seed bank does not usually contain large numbers of Canada thistle seeds [36,184], although there is evidence of seed banking in a coastal British Columbia coniferous forest soil [110], in mature forest sites in central Idaho [117], and in the Delta Marsh in Manitoba [229]. Length of survival is related to depth of burial, with seeds surviving up to 22 years when they are buried more than 8 inches (20 cm) deep [78]. Under more natural conditions of shallower burial and periodic soil disturbance, Canada thistle seeds are more short lived (<5 years), with most seed being lost from the soil seed bank by germination during the 1st year [55]. Seeds that have been in water for several months can still be viable [84]. Donald [55] summarizes the research on seed banking in Canada thistle and the effects of seed immersion in water.

Seedling establishment: Canada thistle seedlings usually start growing slowly and are sensitive to competition and shading [55,89,128]. Seedlings grow poorly in very moist, poorly aerated soils and do not tolerate drought stress [245]. Before seedlings become perennial, they are also highly susceptible to tillage [152].

Asexual reproduction: Vegetative spread of Canada thistle can occur from horizontal extension of the root system, from root fragments, or from subterranean stem tissue [131]. Spread can be rapid when there is little competition, with 13 to 20 feet (4-6 m) of horizontal root growth possible in one season [97,185]. Canada thistle can develop new aerial shoots at any location along the root length, from the original vertical root, or from buds on lateral roots. Within a few weeks of germination, a Canada thistle seedling with at least 4 true leaves can begin producing root buds that can eventually produce new shoots [84]. Buds on lateral roots may form new adventitious shoots as frequently as 0.3 to 1-inch (0.8 to 2.4 cm) intervals [103], although the number of root buds is likely to vary from place to place and year to year [157]. A single Canada thistle plant can potentially produce 26 adventitious shoots, 154 adventitious root buds, and 364 feet (111 m) of roots after 18 weeks of growth [152,157]. It is possible that a colony of male plants would maintain itself regardless of whether it produced fruits [240].

Root buds are inhibited by the presence of the main shoot, primarily due to a competition for water [104], and new root bud growth is highest during late fall and winter months following death of aerial shoots [137]. When the main shoot is removed (e.g. as by mowing) the root buds are released, and new shoots emerge rapidly, especially when humidity is high [104,157]. Wilson [245] found that some 19-day old plants were capable of regenerating top-growth after clipping, and that 40-day old plants could produce 2 or 3 shoots after clipping. Root fragments as short as 0.2 inch (6 mm) and more than 6 weeks but less than 2 years old can regenerate entire plants, regardless of whether they have identifiable root buds at the time [157]. Nadeau and Vanden Born [157] observed that an 18-week-old plant had the potential of producing 930 shoots if its root system was cut into 10-cm-long pieces.

Vegetative spread of Canada thistle may also occur from subterranean stem tissue that can produce shoot buds and adventitious roots at each node. Partially buried stem sections from the postbloom stage survived and produced adventitious roots that over wintered and produced new infestations the following spring [131]. Similarly, Canada thistle can survive disturbance to be part of the early successional community in natural areas by resprouting from buried root and stem fragments [2,45,188,216].

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Life History and Behavior

Reproduction

Biology and Spread

Canada thistle produces an abundance of bristly-plumed seeds which are easily dispersed by the wind. Most of the seeds germinate within a year, but some may remain viable in the soil for up to twenty years or more. Vegetative reproduction in Canada thistle is aided by a fibrous taproot capable of sending out lateral roots as deep as 3 feet below ground, and from which shoots sprout up at frequent intervals. It also readily regenerates from root fragments less than an inch in length.

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Management

Prevention and Control

Management of Canada thistle is very difficult and requires repeated applications of systemic herbicides including products not covered in this guide. Glyphosate is not very effective against it. Other sources will likely need to be contacted for more effective herbicides.

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Relevance to Humans and Ecosystems

Benefits

Cultivation

The preference is full sun, moist to mesic conditions, and a fertile soil consisting of clay-loam. Canada Thistle can grow in drier sites with less fertile soil, but the resulting plants will be stunted. The upper leaves often turn yellow or pale green in response to severe summer heat and drought, and growth will stop. Eradication of this plant is difficult because it can regenerate from small pieces of the rhizomes. The most effective control method involves the application of broadleaf herbicides.

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Importance to Livestock and Wildlife

Livestock tend to dislike and avoid Canada thistle and may also reduce their consumption of desirable plants in the vicinity of Canada thistle colonies [128,146]. Canada thistle can be a minor component in the winter and spring diet of mule deer [12,120]. White-tailed deer forage on Canada thistle in marsh meadows [73]. Thistles (Cirsium spp.) are sometimes eaten by grizzly bear [46]. There are more than 130 species, including pathogens, birds, and over 80 insects, known to feed on Canada thistle [136,161]. Larvae of the painted lady butterfly feed on Canada thistle, but only on an intermittent basis [181,207]. Seeds of Canada thistle are eaten by goldfinches, whose diet consists largely of thistle seeds. Many of the seeds are destroyed this way, but some may pass through the birds unharmed [185].

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Other uses and values

The fragrant flowers of Canada thistle attract honeybees, the primary pollinator for this species. Thistles (Cirsium spp.) are reported to be both edible [176] and medicinal [83]. Canada thistle has been used by native peoples in the northeastern United States in remedies for worms and poison-ivy (Toxicodendron radicans) and was used to make a mouthwash for children, a treatment for tuberculosis (Duke 1986, cited in [83]), and a tonic for gastrointestinal ailments [147]. The roots and shoots of Canada thistle are said to be tender and tasty when taken early in the spring, and were reportedly used as a food in Russia and by North American natives [185]. The roots of Canada thistle, however, may be emetic when consumed (Lewis and Elvin-Lewis 1977, cited in [152]). "Cirsium" comes from the Greek "cirsos," meaning "swollen vein," for which the thistle was considered a remedy [236].

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Risks

Ecological Threat in the United States

Natural communities that are threatened by Canada thistle include non-forested plant communities such as prairies, barrens, savannas, glades, sand dunes, fields and meadows that have been impacted by disturbance. As it establishes itself in an area, Canada thistle crowds out and replaces native plants, changes the structure and species composition of natural plant communities and reduces plant and animal diversity. This highly invasive thistle prevents the coexistence of other plant species through shading, competition for soil resources and possibly through the release of chemical toxins poisonous to other plants.

Canada thistle is declared a "noxious weed" throughout the U.S. and has long been recognized as a major agricultural pest, costing tens of millions of dollars in direct crop losses annually and additional millions costs for control. Only recently have the harmful impacts of Canada thistle to native species and natural ecosystems received notable attention.

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Ecological Threat in the United States

Once established, if conditions are suitable, Canada thistle can form dense stands that shade out and displace native plants, changing the plant community structure and species composition and reducing biodiversity. It spreads rapidly and is very difficult to remove.

Contents

A number of other names have been used in the past, or in other areas including: Canada Thistle,[4] Canadian Thistle, Lettuce From Hell Thistle, California Thistle,[5] Corn Thistle, Cursed Thistle, Field Thistle, Green Thistle, Hard Thistle, Perennial Thistle, Prickly Thistle, Small-flowered Thistle and Way Thistle. The first two names are in wide use in the United States, despite being a misleading designation (it is not of Canadian origin).[6]

Stems are green smooth and glabrous (having no Trichome or glaucousness), mostly without spiny wings. The stems often lie partly flat by summer but can stay erect if supported by other vegetation. The leaves are very spiny, lobed, up to 15–20 cm long and 2–3 cm broad (smaller on the upper part of the flower stem).

The inflorescence is 10–22 mm diameter, pink-purple, with all the florets of similar form (no division into disc and ray florets). The flowers are usually dioecious, but not invariably so, with some plants bearing hermaphrodite flowers. The seeds are 4–5 mm long, with a feathery pappus which assists in wind dispersal.[3][7][8] The plant also spreads underground using rhizomes.

C. arvense is a C3 carbon fixation plant.[11] The C3 plants, originated during Mesozoic and Paleozoic eras, and tend to thrive in areas where sunlight intensity is moderate, temperatures are moderate, and ground water is plentiful. C3 plants lose 97% of the water taken up through their roots to transpiration.[12]

The species is widely considered a weed even where it is native, for example being designated an "injurious weed" in the United Kingdom under the Weeds Act 1959.[18] It is also a serious invasive species in many additional regions where it has been introduced, usually accidentally as a contaminant in cereal crop seeds. It is cited as a noxious weed in several countries; for example Australia, Brazil, Canada, Ireland, New Zealand, and the United States. Many countries regulate this plant, or its parts (i.e., seed) as a contaminant of other imported products such as grains for consumption or seeds for propagation. In Canada, Cirsium arvense is classified as a primary noxious weed seed in the Weed Seeds Order 2005 which applies to Canada's Seeds Regulations.[19]

Control methods include:

cutting at flower stem extension before the flower buds open to prevent seed spread. Repeated cutting at the same growth stage over several years may "wear down" the plant.

Applying herbicide: Herbicides dominated by phenoxy compounds (especially MCPA) saw drastic declines in Thistle infestation in Sweden in the 1950s.[11]MCPA and Clopyralid are approved in some regions.

Orellia ruficauda feeds on Canada thistle has been reported to be the most effective biological control agent for that plant.[20] Its larvae parasitize the seed heads of the plant feeding solely upon fertile seed heads.[21]

Like other Cirsium species, the roots are edible, though rarely used, not least because of their propensity to induce flatulence in some people. The taproot is considered the most nutritious.[citation needed] The leaves are also edible, though the spines make their preparation for food too tedious to be worthwhile. The stalks, however, are also edible and more easily de-spined.[24]

The flower portion is also used by the Cherokee Indians to make blowgun darts. [25]

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Names and Taxonomy

Taxonomy

The currently accepted scientific name for Canada thistle is Cirsium arvense (L.)Scop. (Asteraceae) [42,75,81,92,94,103,107,126,134,177,186,208,232,248]. Canada thistle is extremely variable with regard to leaf division and vestiture,and it has been treated as several species, numerous varieties, or as a singlehighly polymorphic species [81]. Several authors recognize different varieties basedprimarily on differences in leaf morphology [42,75,81,94,232,238]. Voss [232] says it is doubtful that the variety designations aremeaningful, and Cronquist and others [42] state that contemporary Europeanbotanists do not consider described variants of the species to be taxonomicallysignificant.